In recent years, there has been an increasing demand for a sensor network in which real-world information is obtained using sensors and used at a remote location through the network. Existing Internet services are limited to virtual-space services. The sensor network essentially differs from the current Internet in that the sensor network is integrated with a real space. When integration with a real space can be achieved, various services dependent on situations such as time and location can be realized. Traceability is realized by connecting a variety of objects existing in the real space to the network, thereby making it possible to meet social needs for “safety” in a broad sense, needs for efficient inventory control work, and other needs. The sensors directly monitor a real space in terms of the temperature, the degree of soil contamination, the number of engine rotation, etc. and obtained data is shared through the network. Further, a physical action can be performed via an actuator or the like.
A key to realizing a sensor network is employing compact wireless nodes as described in “The Platforms Enabling Wireless Sensor Networks” in COMMUNICATIONS OF THE ACM, June 2004/Vol. 47, No. 6, pp. 41 to 46. Compact wireless sensor nodes require no additional wiring (for power lines and communication lines) and no frequent battery replacement thanks to low-power consumption, and can easily be attached to various things or installed at various locations. As an advantage of compact wireless nodes, wide applications are expected as described below. For example, when compact wireless nodes are installed at buildings, plants, or various electronic devices, it will be possible to sense physical quantities, and to perform remote maintenance/monitoring and automatic replenishment of consumables. When multiple sensor nodes are installed in the natural world, it is possible to perform disaster monitoring and environmental monitoring by sensing signs of a landslide, a flood, and a forest fire before those disasters occur.
FIG. 2 shows a configuration of a typical compact wireless sensor node. A compact wireless sensor node 10 requires no power line connected to an external power supply. Instead, the compact wireless sensor node 10 uses, as its own power supply 11, a small battery built in the node itself or a power source obtained from nature, such as solar power generation, to perform data processing and transmission and reception processing. In order to utilize such limited power as effectively as possible, power required for the sensor node needs to be thoroughly reduced. The sensor node 10 includes a sensor 14, a controller 13 realized by a microprocessor for controlling data transmission and reception, and a radio processing unit 12, all of which have minimum capabilities in order to realize power saving. Sensing data processed in the radio processing unit 12 is sent, via an antenna 15, to another sensor node 10 or to a base station that accommodates the sensor node 10.
FIG. 3 shows the timing of intermittent operation characterizing the compact wireless node 10. In FIG. 3, the horizontal axis indicates time and the vertical axis indicates consumed current. The compact wireless node 10 is periodically activated to be in an operating state 220 (sensing and radio processing), and otherwise in a sleep state 230, thereby performing an intermittent operation for reducing standby power consumption. A method of managing a sensor network through clustering to suppress battery power consumption in the sensor network is described in “Energy-Efficient Communication Protocol for Wireless Microsensor Networks”, written by Wendi Rabiner Heinzelman et al, IEEE Proceedings of the Hawaii International Conference on System Sciences, Jan. 4 to 7, 2000, Maui, Hi. In the method named Low Energy Adaptive Clustering Hierarchy (LEACH), a group (referred to as cluster) is composed of multiple sensor nodes. From the cluster, one sensor node called a cluster head is selected. The cluster head is always activated to play a representative role to relay data sent from another sensor node in the cluster to another cluster or to a base station. Since the sensor nodes other than the cluster head in the cluster do not need to be always activated, the sensor nodes are activated only at their own timing to send information while in a sleep state at the other periods. Thus, standby power consumption can be saved. The cluster head is not fixed but is dynamically selected from among the sensor nodes in the cluster based on the remaining power, or is selected at random, to balance the power consumption in the cluster. As described above, LEACH aims at improving the lifetime of the entire sensor network by dynamically performing coordination in units of clusters.
Although the sensor network aims at improving the lifetime of the entire sensor network, each sensor node with a simplified configuration does not have a countermeasure against service interruption caused by battery replacement or sensor failure.
In the Internet world, a method called load balance is often used to avoid service interruption. Referring to FIG. 4, a description is given to the load balance of a server for performing processing for a particular purpose. Discussed is a case where a user 120 accesses servers 100-1 to 100-3 for realizing multiple identical processings, via the Internet. A load balancer 140 is provided before the servers 100. The load balancer 140 periodically monitors the operating states and the loads of the multiple servers 100-1 to 100-3, provided thereafter. In response to a request sent from the user 120, the load balancer 140 selects, based on a predetermined policy, a server having the lowest processing load or a server having a higher response speed, for example, and performs task assignment. The load balancer 140 does not use a server that has failed unexpectedly or a server that needs maintenance, so the user can always access a server without service interruption.